Conditioning molten metal



Oct. 1, 1946. E. c. LYoNs I CONDITIONING MOLTN METAL Filed Nov. l0, 1944r INVENTOR.

ELLIS C. LTONS 2 Sheets-Sheet A1 Patented Oct. 1, 1946 UNITED STATESPATENT OFFICE CONDITIONING MOLTEN METAL Ellis C. Lyons, Boulder City,Nev., assignor, by mesne assignments, to Reconstruction FinanceCorporation, a corporation of the United States Application November 10,1944-, Serial No. 562,863

'7 Claims. 1

This invention relates to the conditioning of easily oxidized metals,such as magnesium and magnesium base metals or alloys', for casting, andis more particularly concerned with the superheating and subsequentcooling to proper casting temperature ofmagnesium base alloys and likemetals. The invention aims to provide certain improvements inconditioning such metals, particularly in superheating and subsequentlycooling t'o proper casting temperature magnesium base alloys, as well asto provide improved apparatus therefor.

Magnesium base alloys are commonly employed for structural and othermechanical and commercial uses, as, for example, in the automotive andaircraft industries. Among the many alloys of this class may bementioned, merely by way of example, themagnesium-aluminum-zinc-manganese alloys containing 0.1 to 12% aluminum,none up to zinc, none up to 3% manganese and the balance essentiallymagnesium. Magnesium base alloys, particularly those containingaluminum, in the absence -of special treatment, tend to solidify in acoarse grain structure. It is a matter of common knowledge that suchalloys when oi iine grain structure possess superior Amechanicalproperties, enhanced amenability to solution treatment, shorterheat-treating requirements, and improved machinability. It is also amatter of common knowledge that this desirabe iine grain structure canbe obtained by superheatin-g the molten alloy to temperatures far abovethe melting temperature of the alloy, as, for example, by super-heatingat a temperature between 850 and 950 C. According to the presentcustomary practice, the alloy is heated to the superheating temperature,and cooled to the proper casting temperature, generally between 650 and850 C. The superheating, as it is called, imparts to the subsequentlysolidified alloy a relatively vfine grain structure with the attendantsuperior properties hereinbeiore mentioned. The iine grain structuretends to persist, so that castings subsequently produced fromsuperheat-ed alloy .possess a ne grain structure.

The present invention, in one oi its aspects, provides an improvedmethod of superheating and subsequently cooling to proper castingtemperature a magnesium base metal or alloy. In accordance withthisaspect of the invention, molten magnesium base metal is transferred,without Contact with any oxidizing influence (such as air), from beneaththe surface of a body of the molten metal to an elongated conduit havinga superheating section in direct communication with a cooling section.The molten metal is heated in the superheating section to thecontemplated superheating temperature, substantially higher than theproper casting temperature, and in the cooling section the superheatedmetal is cooled to the proper casting temperature. Molten metal ci theproper casting temperature is withdrawn from the cooling section, asrequired for casting or the like, and simultaneously an equivalentamount of molten metal is transferred to the superheating section of theconduit as hereinbefore described. Throughout the operation bothsections of the conduit are led with molten metal, so that at no time isthe molten metal subjected to oxidizing or other contaminating iniluenceduring its transfer to and passage through the conduit. Preferably, themolten metal is transferred to the conduit by the pressure of an inertgas, and the same inert gaseous pressure discharges the molten metal ofproper casting temperature from the cooling section of the conduitsimultaneously with the transfer of an equivalent amount of molten metalto the superhea'ting section ci' the conduit.

In its broader aspect, the invention provides an improved method ofconditioning an easily oxidized metal for casting or the like by trans-'ferring molten metal from beneath the surface of a body of the moltenmetal to an elongated conduit. Thermal agencies operatively associatedwith the conduit bring the molten metal therein to a temperature withina predetermined range. Molten metal of a temperature within that rangeis withdrawn from the conduit as required ior casting or the likesimultaneously with the aforementioned transfer of an equivalent amountof molten metal to the conduit.

The improved apparatus of the invention is particularly adapted forcarrying out the hereinbefore described improvements in conditioningeasily oxidized metals, particularly magnesium base metals.

The novel features of the invention which I believe to be patentablycharacteristic thereof are set forth in the appended claims. Theinvention itself and the construction and operation of apparatusembodying the same will be best understood from the followingdescription taken in conjunction wit-h the accompanying drawings, inwhich,

Fig. l is an elevation, partly in section, of an apparatus embodying oneaspect of the invention, :and

Fig. .2 is an enlarged sectional elevation of typical superheating andcooling units of the apparatus. l

The molten metal to be cast is confined in a basin or crucible 5. Metalmay be added to the basin from time to time, as required, by syphoningor in any other suitable manner. The surface of the molten metal in thebasin may be covered by a protective flux if desired. Alternatively, asshown, an enclosed pot may be used. A pipe B having a valve 'I isthreaded into the cover B for the pot, and serves to introduce an inertgas, such as sulphur dioxide, above the surface of the molten metalunder a pressure sufficiently high to force the molten metal through amolten metal duct 9 to the superheating duct or conduit. The duct 9extends into the body of molten metal in the pot 5 to a desired depth,and is covered, exteriorly of the pot, by thermal insulation I0.

The superheating unit comprises a thermally insulated tank or the like II having an insulated cover I2. The cooling unit comprises a. similarinsulated tank or the like I3 having an insulated cover I4. Preferablythe superheating unit is positioned directly above the cooling unit,although the two units may be positioned side by side or in any othersuitable relative positions. The tank I I of the superheating unit issubstantially filled with a fused salt bath I5, and the tank I3 issimilarly filled with a fused salt bath I5. Pairs of electrodes I1 andI8 extend into the fused salt baths I5 and I8, respectively, and areelectrically connected to a suitable source of electric energy I9through thermostatic control devices 2U and 2 I, respectively, withwhich pyrometers 22 and 23. respectively, are operatively associated, Aswill be understood by those skilled in the art, the electrodes I I andI8 and their associated thermo-static control devices automaticallymaintain the fused salt baths I5 and I6 at predetermined temperatures.

A molten metal conduit 24 is submerged in the fused salt bath I5, and asimilar molten metal conduit 25 is submerged in the fused salt bath I6.The conduits 24 and 25 are preferably in the form of coils, and thelower end of the conduit 24 is connected to and communicates directlywith the upper end of the conduit 25 by a conduit 2B. The upper end ofthe conduit 24 is connected to and communicates directly with theconduit B. The lower end of the `conduit 25 is connected to a discharge`or casting conduit 2'! having a gate or other suitable valve 28. A mold2S, advantageously carried by a conveyor 3G, is positioned under thedischarge outlet of the casting conduit 21. The mold 29 may be of anydesired type for casting ingots, slabs, billets or any other cast form.The conduit 2l' is enclosed by a casing 3|, suitably spaced therefrom,and sulphur dioxide or other suitable inert is introduced through avalved pipe 32 into the annular space between the conduit 21 and casing3| and flows out of the open end of this space over the surface of themolten metal being cast in the mold 29. A hood 33 having a gas exhaustpipe 34 is mounted above the mold 29.

In the usual operation of the apparatus, the fused salt bath I5 isthermostatically maintained at a temperature which heats the moltenmetal in the conduit 24 tothe contemplated temperature of superheat,usually from 85S to 950 C. Molten metal at the temperature of superheatflows through the connecting conduit 25 into the cooling conduit 25. Thetemperature of the fused salt bath I6 is thermostatically controlled tocool the superheated molten metal to the proper casting temperature (e.g. 650 to 850 C.) before it is Withdrawn from the cooling conduit 25.Obviously, the fused salt bath I6 absorbs heat from the superheatedmolten metal delivered to the cooling conduit 25, and the heat thusabsorbed may be sufiicient to maintain the fused salt bath at thenecessary temperature for effecting the contemplated cooling of thesuperheated molten metal. By correlating the heat absorbed by the fusedsalt bath I6 to the heat radiated from the insulated tank I3, as forexample by adjusting the thermal insulation about the tank, little, ifany, additional heat need be supplied to the bath I6. Indeed, thethermal insulation of the tank i3 may be so effective that the fusedsalt bath may have to be artificially cooled in order to cool thesuperheated molten metal to the proper casting temperature. Thetemperature of superheat is uniformly and accurately controlled Withinnarrow limits (e. g. within 5 C. or even less) and the temperature ofcasting is similarly uniio-rmly and accurately controlled Within thesame narrovv limits.

The conduits 9, 24, 26 and 25 constitute a continuous enclosed passagefor the flow of molten metal from the basin E to the mold 29. The moltenmetal in its passage through these conduits is subjected to no oxidizingor other contaminating influence. During its passage through theseconduits, the molten metal is superheated and then cooled to the propercasting temperature. Molten metal oi the proper casting temperature isWithdrawn, by manipulation of the valve 28, as required for filling themold 29 and succeeding molds as moved into position beneath thedischarge outlet of the casting conduit 21. The inert gaseous atmosphereabove the molten metal being cast protects the casting against oxidationuntil it has set or solidified to the necessary extent. rhe gas pressureabove the surface of the molten metal in the basin 5 moves the moltenmetal stream through the various conduits when the valve 28 is open. Asmolten metal of the proper casting temperature is withdrawn from thecooling conduit 25, an equivalent amount of molten metal at thetemperature of superheat" 'flows into that conduit, and simultaneouslyan equivalent amount of molten metal is delivered to the superheatingconduit from the basin 5. The operations ofsuperheating and cooling areappropriately adjusted so that casting is carried out in a substantiallycontinuous manner. Additional metal may be introduced into the basin,from time to time as required, by siphoning or in any other suitablemanner. Supcrheating, vcooling and casting may thus proceed in asubstantially continuous manner so long as the basin 5 contains moltenmetal.

In the heretofore customary practice of superheating magnesium basemetals or alloys, a considerable volume of the molten metal issuperheated in a pot or Crucible, such for example as the basin 5. Thisvolume of molten metal is then cooled to the proper casting temperature,and cast by pouring, ladling or the like. In accordance with theinvention, the molten metal the basin 5 is relatively quiescent, moltenmetal is Withdrawn from the basin and delivered to the superheating unitwithout agitating or disturbing the molten metal in the basin, and issuperheated only as required for the casting operation and hence inrelatively small volume. Superheating proceeds continuously andsimultaneously with casting. Thus, the invention eliminates heatinglarge pots to the high temperatures of superheat, thereby extendingtheir life, and effects a substantial saving in the time formerlyinvolved in superheating and then cooling the molten metal, therebyspeeding up general refinery and casting operations. Superheatingrelatively small volumes of the molten metal, in accordance with theinvention, insures a more uniform product when cast, prevents flux andoxide inclusions in the ingots or other cast forms, and enables theproduction of clean castings of high purity, as the molten metal in thebasin 5 is not disturbed or agitated during the operation.

The coiled forms of the conduits 24 and 25 provide elongated passagesfor the molten metal in the superheating and cooling units,respectively. The conduits have a considerable surface area in contactwith their respective fused baths, thus -facilitating prompt heattransfer. The volumetric capacity of the conduits 24 and 2f: is suchthat an adequate volume of molten metal is continuously superheated andthen cooled as the molten metal ows through the conduits in a more orless continuous manner as required for practically continuous casting.

While I prefer to submerge the superheating and cooling conduits infused salt baths whose temperatures are automatically controlled withinnarrow limits, other heating and cooling agencies may be substituted forthe fused salt baths without departing from the spirit of the invention.In large units, and in smaller units where necessary, appropriate meansmay be provided for agitating or circulatincr the fused salt baths inorder to maintain the entire body of each bath at its contemplatedoperating temperature. Either the superheating unit or the cooling unitmay be used alone, although the full advantages of the invention arerealized by the conjoint use of the two units as hereinbefore described.

I claim:

1. The improvement in conditioning an easily oxidized metal for castingwhich comprises transferring molten metal without contact with air frombeneath the surface of a body of the molten metal to an elongatedconduit, heating the molten metal in the conduit to a temperature withina predetermined range above the temperature of the body of molten metal,and withdrawing molten metal at a temperature within said range from theconduit simultaneously with the transfer as aforesaid of an equivalentamount of molten metal to the conduit.

2. The improvement in conditioning an easily oxidized metal for castingwhich comprises transferring molten metal by the pressure of an inertgas from beneath the surface of a body of the molten metal to anelongated conduit, heating the molten metal in the conduit to atemperature within a predetermined range above the temperature of thebody of molten metal, and discharging molten metal at a temperaturewithin said range from the conduit by the aforesaid inert gaseouspressure simultaneously with the trans-- fer as aforesaid of anequivalent amount of molten metal to the conduit.

3. The improvement in conditioning a magnesim base metal for castingwhich comprises transferring molten metal without contact with air frombeneath the surface of a body of molten metal to an elongated conduit,heating the molten metal in the conduit to a temperature within apredetermined range above the temperature of the body of molten metal,`and withdrawing molten metal at a temperature within said range from theconduit simultaneously with the transfer as aforesaid of an equivalentamount of molten metal to the conduit.

4. The improvement in conditioning a magnesium base metal for castingwhich comprises transferring molten metal by the pressure of an inertgas from beneath the surface of a body of the molten metal to anelongated conduit, heating the molten metal in the conduit to atemperature within a predetermined range above the temperature of thebody of molten metal, and discharging molten metal at a temperaturewithin said range from the conduit by the aforesaid inert gaseouspressure simultaneously with the transfer as aforesaid of an equivalentamount of molten metal to the conduit.

5. The improvement in conditioning a magnesium base metal for castingwhich comprises introducing the molten metal into an elongated conduithaving a superheating section in direct communication with a coolingsection, heating the molten metal in the superheating section to atemperature substantially above the proper casting temperature of themetal, cooling the molten metal in the cooling section to the propercasting temperature, and withdrawing molten metal of the proper castingtemperature from the cooling section simultaneously with theintroduction as aforesaid of an equivalent amount of molten metal intothe superheating section.

6. The improvement in conditioning a magnesium base metal for castingwhich comprises transferring molten meta1 from beneath the surface of abody of the molten metal to an elongated conduit having a superheatingsection in direct communication with a cooling section, heating themolten metal in the superheating section to a temperature substantiallyabove the proper casting temperature of the metal, cooling the moltenmetal in the cooling section to the proper casting temperature, andwithdrawing molten metal of the proper casting temperature from thecooling section simultaneously with the transfer as aforesaid of anequivalent amount of molten metal to the superheating section of theconduit.

'7. The improvement in conditioning a magnes1um base metal for castingwhich comprises transferring molten metal by the pressure of an propercasting temperature from the cooling section by the aforesaid inertgaseous pressure simultaneously with the transfer as aforesaid of anequivalent amount of molten metal to the superheating section of theconduit.

ELLIS C. LYONS.

